1. Field of the Invention
The present invention relates in general to methods and apparatus for making an asphalt-aggregate pavement. In particular, the invention relates to methods and apparatus for transporting the asphalt-aggregate material from the delivery trucks to the screed of a finishing machine.
2. Summary Of The Prior Art
The conventional method of laying an aspahlt-aggregate roadway mat includes the use of a finishing machine and a number of delivery trucks to transport the asphalt-aggregate material from the asphalt plant to the job site. Finishing machines of the type hereunder consideration, sometimes called pavers, are well-known to those skilled in the art. Such a finishing machine has a so-called floating screed at its rear end, usually provided with some form of vibratory means, to form the asphalt mat. The finishing machine also includes a hopper at its front end for receiving the asphalt-aggregate material and suitable conveyor means, usually slat conveyors and screw augers, for delivering the material from the hopper to a position just in advance of the screed.
A typical finishing machine has a hopper with a capacity of approximately 5 tons. A typical delivery truck, usually a dump truck, will have a capacity of approximately 20 tons. The technique for transferring the asphalt-aggregate material from the dump truck to the hopper of the finishing machine screed normally requires the truck driver to position his vehicle immediately in front of the finishing machine, the latter being provided with rollers which engage the rear tires of the truck. During the time that the material is being transferred from the dump truck to the hopper of the finishing machine, the latter is advancing along the subgrade as it forms the asphalt-aggregate mat and in doing so pushes the truck forwardly in tandem therewith.
A number of problems arise when operating the finishing machine and the delivery trucks in the manner just described. Quite frequently, the hopper of the finishing machine will be nearly empty and thus ready to be refilled, but a loaded dump truck will not be available at the job site. The absence of a loaded delivery truck at the required time is most often due to traffic conditions which prevent the delivery trucks from arriving at the job site at the proper time intervals. When this occurs, the finishing machine must obviously stop and await the arrival of another delivery truck.
As is known to those skilled in the art, when a finishing machine stops, even momentarily, the screed will tend to settle into the freshly laid mat. When the finishing machine then commences forward travel, the screen will tend to ride upwardly momentarily thus depositing an excessive amount of material. Consequently, the stopping of the finishing machine causes a depression and bump in the surface of the asphalt-aggregate mat resulting in an uneven pavement surface. Needless to say, it is desirable to produce the smoothest possible surface.
It is often necessary to stop the finishing machine even though one or more loaded delivery trucks are available at the job site. This is so since quite often it is simply impossible for the truck drivers to remove the empty delivery truck from the front of the finishing machine and to maneuver a full delivery truck into a position in advance of the finishing machine before the finishing machine runs out of material. As illustrated in Table I (set forth below), as the paving rate of the paver, in tons per hour (TPH), increases the available truck exchange time decreases.
TABLE I ______________________________________ TRUCK EXCHANGE TIME FOR A CONVENTIONAL FINISHING MACHINE OPERATING DISCONTINOUSLY FINISHING MACHINE WITH 5 TON HOPPER CAPACITY OPERATES CONTINUOUSLY WITH 20 TON CAPACITY TRUCKS 20 TON TIME AT TRUCK PAVING TRUCK PAVER EXCHANGE RATE INTERVAL W/5 TON TIME (TPH) (MINUTES) LIVE HOPPER AVAILABLE ______________________________________ 200 6 4.5 1.5 300 4 3.0 1.0 400 3 2.2 0.8 600 2 1.5 0.5 ______________________________________
As is known to those skilled in the art, at least 1.5 minutes are required to perform the truck exchanging operation. Therefore, it is apparent from the data in Table I that a finishing machine using the conventional supply methods cannot operate continuously at a rate greater than 200 TPH. In order to operate at a higher paving rate and still provide sufficient time to exchange trucks, the paver must stop each time the hopper's supply of paving material is exhausted. This situation is illustrated in Table II (set forth below), where the finishing machine, running discontinuously, can operate at a rate of 800-1200 TPH while still providing the necessary 1.5 minutes for the truck exchanging operation.
TABLE II ______________________________________ TRUCK EXCHANGE TIME FOR A CONVENTIONAL FINISHING MACHINE OPERATING DISCONTINOUSLY FINISHING MACHINE WITH 5 TON HOPPER CAPACITY OPERATES DISCONTINUOUSLY WITH 20 TON CAPACITY TRUCKS 20 TON TRUCK TIME TRUCK EXCHANGE PAVER INTERVAL AT TIME TPH (MINUTES) PAVER AVAILABLE ______________________________________ 400 6.0 2.3 3.7 600 4.0 1.5 2.5 800 3.0 1.0 2.0 1200 2.0 0.8 1.2 ______________________________________
However, this discontinuous operation of the finsihing machine results in an undesirable depression and hump on the pavement mat each time the finishing machine is stopped to perform the truck exchanging operation, as previously explained.
There have been two general approaches in the prior art in an attempt to deal with the problems just mentioned. The first approach involves providing a delivery truck with a very substantial capacity. Representative prior art showing such trucks include U.S. Pat. Nos. 3,647,096; 3,731,825; 3,750,802; 3,794,194; and German Patent Publication No. 22 60 396 (1972), all in the name of John H. Holland. The provision of such large capacity trucks really does not solve the problem since the finishing machine still must be stopped if one of such trucks is not available in a loaded condition at the finishing machine hopper when its hopper is nearly empty. Further, the trucks shown in the aforesaid Holland patents are of the trailer-truck type making it even more difficult for the truck operator to maneuver the vehicle in advance of the finishing machine hopper.
Another approach, in an attempt to solve the finishing machine stopping problem referred to herein, involves dumping of the asphalt-aggregate material on the subgrade in the form of a windrow in advance of the path of travel of the finishing machine. According to this technique, a windrow loader machine, such as the machine shown in U.S. Pat. No. 3,693,512, is provided for picking up the material from the subgrade and for delivering the material into the hopper of the finishing machine.
This windrow approach has not proved to be very satisfactory. Although a delivery truck can normally dump its contents more rapidly in forming a windrow rather than remaining with the finishing machine when discharging the contents directly into the hopper of the latter, considerable skill is required on the part of the delivery truck driver to form a proper windrow. Accordingly, considerable time is still required to discharge the contents of the delivery truck. Thus, the delivery truck may be required to remain at the job site longer than desirable thereby delaying departure of the truck to the asphalt plant for picking up another load of the asphalt-aggregate material. Another disadvantage to the windrow technique results from the fact that the material is dumped onto the subgrade and hence some of the material picked up by the windrow loader may be contaminated with the subgrade material. Moreover, weather conditions can adversely affect the exposed asphalt material that forms the windrow.